1. Field of the Invention
The invention relates to an electrode foil, and more particularly to a composite cathode foil and a solid electrolytic capacitor comprising the same.
2. Description of the Related Art
For solid electrolytic capacitors, a solid electrolyte (cathode) comprises semiconductor oxides such as MnO2, organic semiconductors such as TCNQ or conductive polymers such as polyaniline, polypyrrole, polythiophene or derivatives thereof. Among conductive polymers, poly(3,4-ethylenedioxythiophene (PEDOT) with high conductivity and stability is popular. A conductive polymer solid electrolyte layer was prepared by immersing an element in a solution containing monomers and an oxidizer to generate chemical polymerization or immersing an element in a solution containing monomers and electrolyte to generate electrochemical polymerization. The conductive polymer layer is coated on a dielectric oxide layer over an anode to act as a cathode. Further, charges are led out through a current collector. To achieve complete capacitance exhibition, in addition to filling conductive polymer electrolyte in pores of a porous electrode, the conductive polymer and the current collector should be closely knitted to one another.
A conductive material, for example, metals, conductive metal compounds (metal oxides, metal nitrides and metal carbides), carbon or composite materials thereof is suitable for use as a current collector. In a chip type solid electrolytic capacitor, a conductive polymer electrolyte is coated by a carbon layer and a conductive silver layer and connected with a metal lead. In a wound type solid electrolytic capacitor, a conductive polymer electrolyte is connected with an electrode foil (a current collector, an external cathode end) such as an aluminum foil.
To ensure that the solid electrolytic capacitor is a low-resistance device, conductive layers with high conductivity, compatibility and adhesiveness with one another over an anode dielectric layer are required. Additionally, to increase capacitance, the electrode and electrolyte should be closely knitted to one another. In a wound type solid electrolytic capacitor, opposite to a plain aluminum foil, a porous cathode aluminum foil (current collector) with an enlarged surface area formed by etching is proper, due to requirement for a sufficient contact area between the cathode foil (current collector) and electrolyte (cathode) to enlarge capacitance. An electrolyte, however, does not easily fill in the internal pores of the etched aluminum cathode foil with high capacitance, contrarily reducing capacitance.
JP 2000-114108 discloses using an etched aluminum foil plated with a titanium nitride conductive layer as a cathode foil (current collector) to increase capacitance. However, after a long thermal treatment with temperatures of 100-300° C., the etched aluminum foil plated with the titanium nitride conductive layer is easily oxidized to form an oxide film of titanium oxide and aluminum oxide therebetween. Compared to a conductive polymer solid electrolyte with resistance of 10−2-103 Ωcm, the oxide film has a considerably large resistance, exceeding 106 Ωcm, causing increased equivalent serial resistance (ESR). The same aforementioned result also occurs for use of metals or metal compounds as a current collector.
Using carbon as the material of current collector can avoid oxidation. JP 2007-042732 discloses using a surface enlarged roughened aluminum foil coated with carbon particles as a cathode foil (current collector) to increase capacitance and reduce ESR. Carbon, however, has low adhesion to aluminum foil, causing current collector peeling and increased ESR. U.S. Pat. No. 7,327,556 discloses using an aluminum substrate coated with a carbon layer to enlarge the surface area thereof. An aluminum carbide interposition layer is further formed between the aluminum substrate and the carbon layer to increase adhesion therebetween. However, a long thermal treatment with temperatures of 440-660° C. for over 10 hours under methane and acetylene is required. In addition, special equipment to ensure safety of the high-temperature process conducted with flammable gases is also required. Thus, production efficiency is hindered.